1. Field of the Invention
This invention relates to asbestos/talc compositions intended for high temperature service in glass and steel, aluminum, and other metal manufacturing processes. More particularly, it relates to millboard compositions from which can be produced Lehr rolls which come into contact with the high temperature surfaces of glass and metal sheets.
In glass making processes it is often necessary to continuously move very hot sheets of glass. To accomplish such movement is the function of high-temperature-resistant roller assemblies known as "Lehr rolls." For instance, in the "Fourcault" process, illustrated in FIG. 1, Lehr rolls 10 arranged in a vertical array draw a hot semimolten glass sheet 12 out of a pool 14 of molten glass. As the sheet progresses upward through the vertical Lehr roll array, it is cooled and emerges at the top as a solidified glass sheet ready for cutting. In the more recently developed "float glass" process illustrated in FIG. 2, molten glass 16 is flowed across the surface of a pool 18 of liquid metal inside a high temperature float bath 20. The molten glass 16 forms a sheet which is drawn from the float bath and through an annealing oven 22 by various arrays of Lehr rolls 24. In both of these processes, the Lehr rolls must have the ability to withstand surface contact temperatures of 750.degree.C or greater. They must also maintain a surface texture which is sufficiently smooth such that contact with the Lehr rolls does not mar the surface of the glass sheets. This is particularly true in the float glass process, where the glass sheet emerging from the float bath has an exceptionally flat and smooth surface.
Lehr rolls also find extensive use in the processes for manufacturing plates of steel, aluminum and other metals where high temperature formation of the plates is involved. In some of these processes, the metal plates are annealed in annealing ovens in which temperatures as high as 1200.degree.C are maintained. Lehr rolls are mounted inside these ovens to support and convey the plates through the ovens. In addition to the obvious requirement of high temperature stability under such conditions, surface smoothness of the Lehr rolls is also necessary, for with many of the metal sheets being produced (such as those of stainless steel) good surface finish is a requirement.
2. Description of Prior Art
Rollers containing portions composed of asbestos fibers and binder are disclosed in U.S. Pat. No. 1,854,509. The rollers there described are intended for low temperature service and the asbestos portions are intended solely to prevent heat transfer from metal end plates into the midsection of the roller, which is composed of non-heat-resistant materials. Roll discs composed of long asbestos fibers bound with organic thermosetting resins are described in U.S. Pat. No. 2,987,802. Roll discs composed of a mixture of different asbestos fibers bound with an inorganic binder are described in U.S. Pat. Nos. 3,317,303; 3,334,010; and 3,456,931; use of a mixture of fibers in conjunction with the inorganic binder is said to increase the service life of the rolls at high temperature. A rubber/asphalt sheet composition containing asbestos and a material designated "fibrous talc" is described in U.S. Pat. No. 2,887,428.